With this module you can theme your nodes by interface in a easy user friendly way (unlike panels, which can do pretty much what ds can but with a million clicks more).
The display suite is collection of modules all sharing the same api, the modules that implement the api are:
For example if you want nodes with cck fields styled and pull views according to the defined buildmode you need to install display suite, node displays and node displays contributions.
How it works
It divides the $content variable in your node tpl into regions like left-right-header-footer etc. Every CCK field on the node can be dragged into a region using the interface. Other fields like the fivestar widget forexample can be dragged too using the nd_contrib module.
You can have your own modules exposing their widgets to ds. It saves lots of theming work. Often you have a multiple lists of the same node types on a site but they are themed differently. With ds you dont have to touch your tpl files. Just created a new display and apply it to your view, that's right views support is present.
It is an alternative way of theming. When new fields are needed they are exposed to the display suite using the interface by creating new fields (even with php code). I would not recommend using php in interface defined fields because it is more difficult to maintain. In stead I would recommend to implement hook_ds_fields() and maintain your fields in code.
The whole approach is a very nice process for the designer aswell as for the developer. The developer doesnt have to worry about the correct html output and doesnt need to move around html in the tpl files anymore. the designer can just concentrate on his css and doesnt need the developer to change something in the tpl.
Also flexibilty is enormous, with just some clicks, the theming of a node can be changed very easy, maybe some css is required but again no messing around in the tpl. So no more panic if the client wants that image left instead of right, a few clicks and the problem is solved. It is so easy to use that you can allow clients to use it without having to worry they ll break the site.
It does generate a lot of divs and some themers have complained about it being difficult to maintain overview.The best way to explain to themers is that they have to see the output of the display suite as a mini page tpl. Analogic to the page tpl which has regions that collapse whenthey are empty and take a percentages of the space when they contain blocks, the $content has node-regions which will show up when fields are dragged into it and colapse when they are empty. It is true that the first project has a learning curve for both themer and developer.
Also some SEO people have complained about the "excess" div problem not being SEO friendly. Personaly I feel it doesnt add up to the advantages it brings to your site. Installing this module will save you some much time in the long term that you be able to use that time to write great content which is a 1000 times more effective to make your site show up in search results than eliminating a couple of divs and sacrificing the flexiblity of changing your displays.
The trees module is based on the book module. Like the book module the trees module allows users to structure nodes in a hierarchy or outline. The main difference is that the trees module supports adding one node to multiple trees which the the book module does not. The output is also different. The book module provides a navigation, the tree module does not. Instead it produces for each tree a set of blocks, being the parents of a node, the siblings and the children.